docs: def last_day(year=_year, month=_month):
      """
      get the current month's last day
      :param year:  default to current year
      :param month:  default to current month
      :return: month's last day
      """
      last_day = calendar.monthrange(year, month)[1]
      return datetime.date(year=year, month=month, day=last_day)